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1999 Sea-Doo Challenger 1800 Rotax 787

Derek81

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Water Crafts
Sea Doo challenger 1800
#1
Hey guys new to the forum and located in Hampton roads Virginia. I have a 1999 Sea-Doo Challenger 1800. STBD engine is running rough, back firing coming from AFT carbonator on the rotax 787, when I remove the intake box you can see the aft carb sparking. Exhaust is pumping, but when it back fires it sucks air briefly. Cleaned the rave valves hoping to fix the issue, they had tons of carbon build up, did not fix it the issue. Checked the tourque and the exhaust system, everything was tight. Changed the spark plugs, doing the same thing, checked fuel and it's good, port engine running fine. Any thoughts?
 


Derek81

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Water Crafts
Sea Doo challenger 1800
#4
Just bought it a month ago from the original owner in Mineral, VA. I texted just now and he said he's never cleaned them.
 

JeremyD615

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Water Crafts
1995 Sea Doo Speedster
1995 Sea Doo XP
1996 Sea Doo XP
#5
They need to be cleaned and rebuilt with GENUINE Mikuni rebuild kits and new needles and seats. OSD Parts is a great source for the correct factory parts. If you buy aftermarket carb parts, you will have further problems and wind up doing the whole job a second time. There’s a carb rebuild sticky in the 2-stroke pwc section that will walk you through the process.
 

Derek81

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Water Crafts
Sea Doo challenger 1800
#6
Thanks Jeremyd615. How challenging is the removal, rebuild process? Will it need to be professionally tuned afterwards? Never messed with carbs before....
 

JeremyD615

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Location
Nashville, TN
Water Crafts
1995 Sea Doo Speedster
1995 Sea Doo XP
1996 Sea Doo XP
#7
It’s really not difficult at all. It’ll take a couple of hours the first time, but if you follow the instructions in the sticky it’s really a simple process and you’ll be fine with the factory settings...
 

Derek81

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Water Crafts
Sea Doo challenger 1800
#8
So I called a Sea-Doo repair shop today and described to them what the boat engine was doing and was told it is a bad rotary valve. Does that sound right to y'all?
 

JeremyD615

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Location
Nashville, TN
Water Crafts
1995 Sea Doo Speedster
1995 Sea Doo XP
1996 Sea Doo XP
#9
It’s possible, but without them seeing the boat to diagnose, it sounds like they’re just planning on charging you for a more expensive repair. Get the carbs sorted out first, as they need to be done either way. If you still have the problem then we can diagnose further...
 

Derek81

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Water Crafts
Sea Doo challenger 1800
#10
Start up question. When I start the left engine (out of water) it shoots up to 3000 rpms and stays there. The right engine (out of water) 1200-1500 rpms. Is this normal?
 

mikidymac

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1996 XP
1996 HX
1991 Superjet
2001 Superjet
#11
Both engines should be 3000 out of the water. The load on the pumps in the water will bring them down to 1500.
 

JeremyD615

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Location
Nashville, TN
Water Crafts
1995 Sea Doo Speedster
1995 Sea Doo XP
1996 Sea Doo XP
#12
No, they should both idle at 3000 rpm out of the water. That may just be an idle adjustment issue though...
 

Derek81

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Water Crafts
Sea Doo challenger 1800
#13
Ya I thought it was a little weird only one was doing that, but it has done that since I bought it.
 

Derek81

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Water Crafts
Sea Doo challenger 1800
#14
Thanks for the advice! Adjusted the engine idle speed screw. Both now at 3000 rpm
 

Derek81

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Water Crafts
Sea Doo challenger 1800
#15
Just bought a compression test set. All cylinders except the one STBD aft one read 150psi, the backfiring one reads 5psi. Suggestions?
 

mikidymac

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1991 Superjet
2001 Superjet
#21
Cylinder was lean. Probably from dirty carbs and or fuel system.

Depending on the hours the crank might not survive a new top end only.
 

Derek81

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Water Crafts
Sea Doo challenger 1800
#24
Rebuild completed. All 4 cylinders read 150psi. Fires right up. Can't wait to test it in the water. Recommendations for break in?
 

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Derek81

Active Member
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Water Crafts
Sea Doo challenger 1800
#25
Hey guys, I was reading some post from this forum on 2-stroke oil. I went to West Marine and they told me to fill it up with West Marine TC-W3. From what I'm gathering from this forum is TC-W3 can kill an engine and I need to be using an API-TC synthetic one. If this is true, any knowledge out there on the best way to drain and flush?
 

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